Toolmarks and Ballistics

Toolmarks result when a hard tool contacts a softer object, leaving marks or striations. Expert toolmark testimony is based on the idea that each set of markings is unique. Testimony about a variety of tools, such as screwdrivers and crowbars, has been admitted. Ballistics relates to firearms and is a form of toolmark comparison. Weapons are a prime source of evidence, and experts try to identify a particular gun as the source of bullets or cartridges from the crime scene or other ammunition components. Most firearms have a groove in the barrel, called rifling, to cause the bullet to spin for accuracy. The rifling leaves distinctive marks on the bullet. Marks are also left by the firing pin on the cartridge and by the ejector mechanism as it removes the cartridge. Using a dual microscope, examiners compare the marks on bullets, cartridge cases, and expended shells to a test firing of the suspected gun. Firearms comparison testimony has been routinely admitted in court. But the problem with ballistics is similar to the issues concerning other pattern identifications. The assumption of uniqueness is questionable, the standards for comparison are not well established, and the conclusions are subjective. Recent advancements in ballistics in the development of computerized databases of firearms and ammunition and in the development of three-dimensional imaging processes may provide a more scientific basis for ballistics testimony.